Catalytic converters are conventionally included in the exhaust system of automotive vehicles to reduce the level of pollutants discharged to the air. While it is generally believed that the catalytic converters used today perform satisfactorily once their light-off temperature is reached, a pollution problem exists during the light-off period. For example, it has been determined that 80% of the pollutants exhausted to the atmosphere from an exhaust system which includes a catalytic converter are formed during the light-off period. As used herein, the light-off temperature is the temperature at which a catalytic converter catalyzes the reaction that takes place in the converter with the exhaust gases. The catalytic light-off period is the time required for the catalytic converter to reach its light-off temperature.
If the heat of exhaust gases traveling from the engine to the catalytic converter can be retained for a longer period of time than in conventional exhaust systems, the time required for the light-off temperature to be reached will be reduced. This would then reduce the duration of high pollution, and in turn reduce the amount of pollutants released to the atmosphere.
Attempts have been made in the past to develop insulated exhaust systems. Double exhaust pipes have been suggested, comprising spaced inner and outer pipes. Although this reduces the amount of heat loss, it is not enough to appreciably retain heat at the level required for optimum catalytic converter operation.
Another suggestion is found in U.S. Pat. No. 4,345,430, issued to Pallo et al. In that patent a double pipe system comprised of inner and outer corrugated metal tubes is disclosed. In addition, the use of insulation between the inner and outer tubes is suggested. There is no appreciation in the Pallo et al patent or in other exhaust pipe designs, however, of the further problem of increased aging of the catalytic converter. Each catalytic converter is designed to most efficiently operate not only above a certain minimum temperature, but also below a certain maximum temperature. When operating temperatures exceed this maximum temperature the catalytic converter is subject to accelerated or increased aging, which in time reduces the effective life of the catalytic converter.
While the use of an insulated exhaust pipe to retain the heat of exhaust gases reduces the light-off period and is thus beneficial in reducing the amount of pollutants discharged to the atmosphere, it also tends to reduce the life of the catalytic converter by delivering gases at a temperature greater than the maximum desired operating temperature. It would therefore seem that the two goals of achieving a short light-off period and a long operating life for a catalytic converter are mutually exclusive and cannot be met in any particular automotive exhaust system.